Automate PR checking with Danger

http://danger.systems/ cc @orta

### Warnings

- [ ] Non-final class in examples
- [x] [Swiftlint](https://github.com/ashfurrow/danger-swiftlint) for examples
- [ ] Markdown verification using [prose](https://github.com/dbgrandi/danger-prose) (incl. link checking)
- [ ] ObjC classes without `IG_SUBCLASSING_RESTRICTED`
- [x] ~Big PR~
- [x] ~No CHANGELOG.md entry for any change~

### Failures

- [ ] New CHANGELOG.md entry when `.h` file is changed
- [ ] Facebook copyright in headers
- [ ] Changes to restricted file types (`.lock`, `.xcconfig`, etc)
- [ ] **Always fail**: "Merging should be done internally"
- [x] ~Only allow editing `/docs` from admins~
- [x] ~WIP pull requests~

Any other ideas?